Manchester encoding relates to a technique of binary data encoding. Basically, each bit is represented by two phases equal in length, but opposite in logic value. In other words, each bit is equally divided into two phases that are opposite in sign. A binary logic ‘1’ is commonly represented in Manchester mode by a logic low phase followed by a logic high phase, and a logic ‘0’ is the opposite. Some encoding systems exist that consider the mirror image signal to be true, i.e. in which a logic ‘1’ is represented by a logic high phase followed by a logic low phase, and a logic ‘0’ the opposite. Manchester encoding has several performance advantages, including the absence of a DC signal component and the ability to recover a clock signal from the encoded data signal.
Bit stream regeneration and data recovery from a serial Manchester encoded bitstream can be difficult. In most systems, a receiver will recover the data rate clock from a preamble, which is a predefined initial pattern composed of a series of logic ones followed by logic lows. After the preamble is received, the receiver performs data recognition by looking for a Sync Word in the bitstream, which is a predefined pattern known to the receiver whose pattern and length may vary from one system to another. All data in the bitstream following the sync word is considered to be the data payload of a packet. The packet is the combination of the preamble followed by the sync word and the data payload. When data is encoded in Manchester mode, it is crucial to know where the data starts in the bitstream in order to determine the leading phase. Otherwise, Manchester to binary data decoding errors will lead to loss of synchronization and eventually packet loss.
Conventional preamble detectors and clock recovery mechanisms working on the Manchester encoded data typically cannot supply the information on the leading Manchester phase discovery. An example of a conventional approach that concentrates on clock recovery by oversampling the bitstream is Phase ambiguity resolution for Manchester-encoded data, U.S. Pat. No. 5,224,126 to Myers et al. An example of a conventional system that focuses on edge detectors for clock recovery is Decoding method and Manchester decoder, U.S. Pat. No. 7,133,482 to Poletto et al. A conventional system that utilizes phase value correction according to signal strength and data recovery based on phase duration to do clock recovery is System and method for decoding Manchester data, U.S. Pat. No. 6,977,973 to Wiggins. Additional examples of convention systems are: U.S. Pat. No. 3,491,202 for Bi-Polar Phase Detector and Corrector for Split Phase PCM Data Signals to Bailey et al.; U.S. Pat. No. 4,733,404 for Apparatus and Method for Signal Processing to Ostoich; and U.S. Patent Publication No. US20030227987A1 for Decoding Method and Manchester Decoder to Poletto et al.